Cloud watching goes hi-tech

Researchers in Northern Ireland have built an instrument that could significantly improve the imaging of clouds from space, leading to more accurate weather forecasts and climate models. The electronic device will give meteorologists and climate scientists access to previously undetectable thermal emissions from clouds, which could reveal valuable information concerning the formation of rainfall and the Earth's energy budget. The researchers have filed for a patent for the device, which will be used by the European Space Agency (ESA) in a number of upcoming missions.

With the continued uncertainty surrounding the effect of clouds in climate models, satellite instruments are playing an increasingly important role in climate science. Space-borne remote sensing instruments, however, have been limited by the fact that they can only detect either vertically or horizontally polarized components of thermal emissions from gases in the Earth's atmosphere – but not both at the same time.

Head in the clouds

Robert Cahill and his colleagues at Queen's University have overcome this problem by designing an electronic filter that can detect thermal emissions up to a very high frequency, regardless of how they are polarized. It consists of two rectangular loops of highly conductive metal embedded into the surface of a silicon wafer. The outer loop begins to conduct when excited by horizontally polarized waves, while an inner loop will only conduct when excited by vertically polarized waves.

The instrument, called a dual-polarized Frequency Selective Surface Filter (SSF), was constructed using silicon rather than metal due to the material's higher tensile strength. It is designed to operate in the 250–360 GHz range but the researchers are also developing an SSF to operate at 664 GHz – the highest dual-polarization detector ever produced. One of the main advantages of SSF over alternative detectors is that it is freestanding, which means that it can be transferred between instruments in a range of different missions.

Cahill told physicsworld.com that, having tested the device under a range of physical conditions, his team is about to begin the first high altitude trials later this year. "We have spent the past few years working on the engineering aspects of the device and we are confident that the present design can withstand very extreme physical conditions, such as those experienced at a space launch," he says.

In developing the SSF to function at much higher frequencies, the Queen's University researchers have been working with the UK Centre for Earth Observation Instrumentation – an organization with the broad goal of improving the quality of instrumentation in Earth observation. The researchers have also secured a contract with the ESA to provide filters for a series of meteorological satellites that it plans to launch between 2018 and 2020.